Conventional methods of monitoring tags on moving objects, such as shipping containers, utilize frequent transmissions reporting an “all's well” message to a security monitoring centre. With this arrangement, an alarm condition may be detected by the security centre either through reception of an alarm message from the tag, or a failure to receive the “all's well” message. The protection provided by this conventional method may be easily circumvented by thieves blocking the radio transmission of an alarm message such as by moving the object and its tag into a tunnel or inside a building. The security centre may further be lulled into a false sense that all is well by thieves re-broadcasting previously recorded “all's well” transmissions. Even without a rebroadcast of the “all's well” signals, the thieves have at least the interval between regular transmission reports to work without detection.
To protect against these kinds of attack, another conventional method requires the tag to report its status continuously, or very frequently. This approach wastes the radio spectrum resources and may therefore limit the number of tags that can be monitored with a limited amount of radio spectrum or communications channel capacity, and also rapidly exhausts the battery in the tag due to the frequent radio transmission. Under normal circumstances, most of the transmissions contain the “all's well” condition and most of the communications traffic is therefore redundant and unnecessary. The frequent transmission also makes the system more sensitive to the normal blockages of transmission that occur in the course of a tagged object's journey. It is difficult to set a tolerance to distinguish natural (i.e., normally occurring) message disruptions and a security alarm event that appears as a message disruption. If the tolerance is set too short, many normal blockages will trigger false-alarms, and if the tolerance is set too long, a deliberate blockage may be assumed to be a natural occurrence and may be dismissed or at least delay detection.
Improvements to conventional mobile tag monitoring systems are therefore desirable.